According to one aspect of the invention there is provided a control system for a multi-ratio gearing for automatically causing changes between ratios of the gearing, in response to variations in rotational speed in the gearing, in which a driver-operated control is provided for biassing the control system to raise or lower the threshold rotational speeds at which changes are initiated, and the resultant threshold speeds are unaffected by the control (i.e. the throttle pedal) for power applied to the transmission over the normal range of movement of the power control, the selected setting of the driver operated control being effective over a range of inter-ratio changes.
British Patent Specification No. 1,444,424 discloses a shift pattern control device in which pressures corresponding to engine output torque and to vehicle speed are applied to opposite end faces of a shift valve member. A driver-operated control enables the driver to vary the shift points to some extent but at all times the shift pattern control device is responsive to engine output torque.
It can be shown that when movement of the throttle or power control member has such an inseparable and important influence on the gear shifting threshold, some very undesirable and even dangerous effects can result when the throttle has to be suddenly shut or reduced. For example, when approaching a roundabout or other temporary restriction on an upward gradient with the throttle open and the transmission operating in a low or intermediate ratio, the driver would prefer to retain the low or intermediate ratio while negotiating the restriction, but because he wishes to reduce speed he must raise his foot from the throttle pedal (possibly even to apply the brake pedal) and consequently the threshold is changed to the extent that the transmission shifts to a higher ratio. When he again opens the throttle, for example to accelerate out of the roundabout, the transmission is compelled to shift down again to a lower gear ratio.
Apart from the fact that gearshifting contrary to the wishes and expectations of an experienced driver of a manually-shifted transmission are disconcerting, it can also result in impulsive changes in tractive effort which impair adhesion to the road surface at a time when a heavily loaded vehicle may be subjected to sideways accelerations due to the curvature of the track, as when negotiating a roundabout.
On icy or otherwise slippery surfaces the danger of skidding is further enhanced. In such circumstances if opening the throttle precipitates a downward gearshift, there is great danger that the extra tractive effort suddenly released will cause the driving wheels to slip, and the increases rotational speed of the transmission due to slip will bring about a series of upward shifts, thus further increasing the slip velocity at the wheel treads and so aggravating the situation.
Such problems are mitigated by this aspect of the invention, an essential feature of which is that movement of the throttle pedal in any part of its normal driving range has no influence on the gearshifting threshold, but instead, the driver has a separate member for varying the threshold in terms of vehicle speed, which is not in any way influenced by the position of the throttle pedal.
According to another aspect of the invention, there is provided a control system for a multi-ratio automatic transmission for automatically causing changes between ratios in a range of ratios in response to variations in an output signal from an output-speed-responsive governor of the transmission, wherein the strength of the output signal from the governor decreases with increasing output speed and the control system is arranged to respond to the decreasing signal strength by correspondingly upshifting the transmission and to respond to absence of signal by upshifting the transmission to the highest ratio in the said range.
British Patent Specification No. 1,514,972 discloses an electronic control system including an evaluating circuit for the output signal for an engine-speed-detecting governor. The governor produces an output proportional to engine speed and thus, in any particular gear ratio, proportional to output speed. The evaluating circuit produces from the governor output an output inversely proportional to the governor output. However, the evaluating circuit is located in an electronic module including the other control circuits for the transmission. Accordingly any failure of the governor or its connection line to the module must result in downshifting of the transmission to or towards the lowest ratio. If this were to occur at high vehicle speed, the result could be disastrous for example by gross overspeeding of the engine or by causing skidding due to the tendency of the lower gear to lock the driving wheels.